To reduce fuel consumption, emissions of harmful substances and noise levels in vehicular traffic, start-stop systems in which when the vehicle is stationary the internal combustion engine is automatically stopped and then automatically started again as soon as a wish to drive on again is recognized, are gaining importance. To enable this function in an automatic transmission in which, to supply the shifting elements with pressure and cooling oil, a hydraulic pump is driven by the internal combustion engine, the shifting clutches or shifting brakes required for a starting gear must already be active or even closed when the internal combustion engine is stopped or at least at a time very close to the re-starting of the internal combustion engine. In this way it can be ensured that a conventional hydraulic pump is capable, during a quick engine start when the hydraulic fluid volume demand is only small, of supplying the shifting elements with sufficient oil pressure for rapid torque transmission. Otherwise, starting up for example after a traffic-light shift would be delayed and the start-stop function would be impractical and uncomfortable.
However the shifting pistons of the shifting elements, which are usually spring-loaded in the opening direction, open completely as soon as the hydraulic pressure falls when the engine and pump are at rest. A considerably more powerful and as such oversized hydraulic pump, which could build up the necessary pressure quickly enough for starting when the shifting piston is open and has moved back to a rear end position, would eliminate the fuel consumption advantage of the start-stop system again due to its permanently higher power uptake during driving operation, and would therefore have a rather counterproductive effect.
In order nevertheless to enable start-stop operation with an automatically controlled transmission, in particular, an automatic transmission of planetary design, it is known for example from DE 10 2006 012 838 A1 and DE 41 34 268 C2 to use an auxiliary pump with its own electric drive which operates when the engine is stopped, in order to maintain at least a permanent feed pressure for the shifting elements provided for starting.
The disadvantages of an auxiliary electric pump are the structural complexity and costs involved, the continuous power uptake and the acoustically perceptible pump operation of the auxiliary pump when the engine is at rest.
Furthermore a hydraulic impulse storage device (HIS) is known, which supplies the shifting elements of a transmission required for starting with hydraulic oil. WO 2007/118 500 A1 shows an impulse storage device of this type in the form of a piston store. During driving operation the piston store is filled with oil against the force of a spring, and mechanically locked. When the engine is restarted the lock is released, for example by means of an electromagnetically controlled actuator, and the stored oil is expelled by the spring-loaded piston and fed into the hydraulic system to assist the pressure build-up of the hydraulic pump and the pressurization of the corresponding shifting elements. This takes place so quickly that when the engine starts the shifting elements concerned are already active, so that the driver does not perceive any delay in the build-up of force in the drive-train. Compared with an electric pump such an impulse store is cheaper and more economical to operate. The disadvantages of an impulse store are the necessary structural complexity and fitting space it demands, and the corresponding control effort involved.
From DE 10 2006 014 759 A1 a hydraulic control device is known, with which the shifting elements relevant for a start-stop function are prevented from opening by hold-back valves when the engine is at rest. The hold-back valve is arranged as a pressure-limiting means in a hydraulic branch between a shifting element valve and a shifting element piston space in order to prevent empty running of the piston space in an active engine-start automatic mode, so that at least a residual volume remains for compensating a ventilation play. For this purpose the hold-back valve closes at a defined residual pressure of the hydraulic pump switched off when the shifting element is closed, which is somewhat lower than a shifting element filling pressure.
The disadvantage in this case is that the control device is better able to maintain a required minimum pressure for a given time in shifting elements attached to a housing with statically pressure-tight oil ducts, i.e. in shifting brakes. In contrast for rotating shifting elements with dynamic, leak-prone seals, i.e. shifting clutches, with comparable seal complexity the control device is in any case suitable for short engine stop times. Besides, the structural and control complexity and cost of the hold-back valves are not inconsiderable.
From DE 10 2007 003 922 A1 a shifting element of an automatic transmission is known, in which a pressure compensation space of a shifting element piston is designed as a dynamic restoring means. The pressure compensation space compensates the control forces on the piston usually produced centrifugally by rotating oil in the piston pressure space, and in addition produces a resulting restoring force which replaces a conventional restoring spring. However, in contrast to a restoring spring the restoring force of the compensation space only acts on the piston when the shifting element is rotating. When the internal combustion engine and transmission are stopped, the piston no longer rotates when the vehicle is at rest. Consequently, despite the falling hydraulic pressure, owing to the absence of the restoring force the piston remains in a forward end position, i.e. almost fully activated. On re-starting, only a small oil volume then has to be replaced, so that the shifting pressure required for a starting process is available in a time short enough for start-stop operation.
The disadvantage in this case is that for the formation of a pressure space that will have sufficiently reliable restoring characteristics with all the hydraulic fluid properties that may occur, a suitably sized structural space, in particular one or more annular hollow spaces, are needed. However, in modern transmissions with in any case restricted structural space and sometimes with design limitations, the space required is not always available. Moreover, the pressure compensation space must in each case be accurately matched to the shifting element concerned, which entails corresponding effort and expense.